This invention relates to a permanent magnet motor rotor arrangement which increases motor or generator efficiency and can withstand high impact shock loads and high centrifugal loads. The permanent magnet motor rotor provides for inexpensive and precise positional control of rotor components.
Conventionally the most commonly used integral horsepower motor rotors have a squirrel cage construction and are used in alternating current (AC) induction motors. AC and direct current (DC) generators typically include wound rotors. DC motors usually include a commutator and rotor windings. Each of these motors or generators have rotors which differ significantly from a rotor including permanent magnets. In addition, each of these rotors develop a rotor magnetic field by electrical current flowing through the rotor. As a result, all these rotors are excessively large, less efficient, more difficult to cool and of complex construction.
Heretofore, rotors including permanent magnets utilized curved magnets which were fixed with adhesive to the periphery of the rotor. Other permanent motor rotors, typically much smaller in size, utilized magnets embedded in the steel rotor core. In that case, stacks of rotor laminations which form pole pieces are generally secured to the rotor using threaded fasteners or dovetails. Use of the dovetails or fasteners typically increases rotor cost and adds excessive parts to the rotor assembly.
Unfortunately, precise location of rotor pole pieces in permanent magnet motor and generator rotors is difficult to achieve and as a result such rotors are usually noisy. Such precision is necessary when using the rotor for applications where torque fluctuations and cyclical radial loads must be kept to a minimum. Moreover, the inability to provide precise radial position and angular orientation of rotor parts can degrade overall rotor or generator performance, cause unacceptable vibration levels and reduce efficiency.
In addition, loosening of the rotor assembly can occur during operation due to normal motor or generator vibration. Loosening of parts can degrade performance or even cause mechanical damage to motor or generator parts. Vibration may also loosen and cause failure of adhesive bond lines which can result in release of the permanent magnets installed on the motor rotor. Degraded performance or mechanical damage may result from magnets which are loosened and separate from the rotor.
The use of adhesives or threaded fasteners to retain magnets on the rotor core makes the magnets susceptible to separation from excessive centrifugal or high impact shock loads imposed on the motor rotor. Moreover, motor or generator heating and environmental conditions degrade the integrity of the adhesive used to secure the magnets to the rotor, potentially leading to eventual magnet separation and rotor failure.
To help retain surface or adhesive mounted permanent magnets, a thin retaining cylinder usually of metallic or wound fibrous construction is employed. The use of such a thin retaining cylinder or can has detrimental effects on machine performance and efficiency. In addition the required cylinder thickness for a large or high speed motor or generator makes the use of such a can for these applications impractical.
Permanent magnets installed in conventional rotors, whether embedded therein or affixed with adhesive, are susceptible to damage and/or demagnetization from overheating. Unfortunately, the magnets are directly exposed to heat effects associated with air gap surface losses, eddy current heating and heat associated with mechanical vibrations induced from air gap harmonics. Heating these magnets near their Curie temperature can cause demagnetization and result in performance loss. Moreover, shorts in simple turn-to-turn or phase-to-phase stator windings may produce dramatic heating of magnets installed in conventional rotors and thus lead to demagnetization.
Further, conventional permanent magnet rotors offer little or no protection from magnet damage or demagnetization under severe operating conditions or common motor and generator casualties. The magnets are susceptible to physical damage because they are usually located on, or have at least one side totally exposed to the rotor air gap surface. Accordingly, imposed shock loads, high vibration levels or mechanical failure of some other closely aligned motor or generator part can result in physical impact to the magnets thereby damaging the magnets.
Typically, permanent magnet rotors which utilize embedded magnets allow the pole pieces to bear directly onto the magnets. Since the magnet material is brittle, this precludes the use of these magnets as reliable stress bearing structural members in many applications, particularly where large rotors are required. Moreover, permanent magnet rotors generally use only single magnets to establish rotor poles. As the size of a permanent magnet rotor increases, single magnet configurations are more susceptible to physical damage from bending, torsional and shear stresses because, as previously discussed, these magnets are very brittle and do not contain the physical strength associated with other metallic rotor components.
In addition, the single magnets used to create poles in conventional permanent magnet rotors may become difficult and hazardous to handle as the rotor size increases. Typically, permanent magnet rotors utilize powerful rare earth magnets that have strong magnetic fields. Handling of such physically large magnets, each with a large magnet field requires development of special tooling and procedure to handle the magnets. Working with large magnets in the vicinity of surrounding ferromagnetic material may also pose safety hazards to personnel.
Further, the difficulties in manufacturing and magnetization of large size single magnets limits the size and ratings attainable in conventional pulse modulated motor and generator designs.